1. Field of the Invention
The present invention relates to footwear, and more particularly, to an athletic shoe having improved cushioning and stability.
2. Description of the Prior Art
It is known in the prior art to provide athletic shoes with a midsole made from a foam material, such as polyurethane, designed to provide for cushioning against impact, that is, attenuation of the applied load. The polyurethane materials which have been used are non-microcellular, having a non-uniform cell structure. These foam materials have a stiffness (k) which varies in dependence upon the applied load. At lower loads, the foam material is only slightly compressed, and has a low stiffness. As the applied load increases, the compression of the cushioning material increases as well, increasing the stiffness. Eventually, the cushioning material will be compressed to a maximum level such that a further increase in the applied load will not cause the material to be further compressed. At this point, for purposes of the maximum loads applied to midsoles, the stiffness of the material will approach an infinite level, that is, effectively no cushioning will be provided.
In general, during footstrike, the initial contact is made at the rearfoot lateral location, with the foot rolling towards the forward or anterior, and medial locations. The applied load increases until the maximum load is achieved, generally beneath the calcaneous. Since the magnitude and location of the applied load are not constant, it has been difficult to construct the midsole to provide a desired level of cushioning throughout the ground support phase, which includes the breaking phase and the propulsion phase, by using conventional non-microcellular polyurethane foam cushioning materials.
For example, a midsole having a predetermined thickness and therefore stiffness (at a given load) could be utilized. The stiffness may be appropriate for the range of loads experienced at the lateral rear of the shoe during footstrike. That is, at that location, the load may not exceed a level which causes maximum compression. However, at the location beneath the calcaneus, the load may exceed this level, the stiffness will approach infinity, and the wearer will experience a sudden loss of cushioning known as bottoming-out. Alternatively, if the material and thickness are designed to compensate for the maximum load, the initial stiffness experienced at the lateral rear will be too high. In addition, the thickness of such midsoles increases the weight of the shoe and reduces rearfoot stability, precluding their use in athletic shoes.
Furthermore, in prior art shoes, a particular level of midsole stiffness would be selected for a given shoe based upon the likely weight of a person wearing a given shoe size, and perhaps, the loads expected to be produced during the activity for which the shoe is designed. However, the midsole stiffness could not be adjusted to take into account weight variations between people having the same shoe size. In addition, even if a stiffness were achieved which was appropriate for a given wearer performing a given activity, the stiffness could not be adjusted so as to provide an appropriate level for other activities having a different range of expected loads. For example, if a shoe were designed for running, even if the stiffness was appropriate for the weight of an "average" person having a particular shoe size, it would have a stiffness which was greater than desired for the loads expected during walking by the same "average" weight person. In addition, the shoe would be either overcushioned or undercushioned for a person having a smaller or greater than average weight, respectively.